Hi Cleyton
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When you create a new theme, simply select the background to be transparent, instead of a color or image.
The video playing is a bit more tricky... There are 2 ways to do it...
1. Use the webkitplayer (requires transparent theme):
Using the webkitplayer you can use "Live backgrounds" as described here: http://manual.openlp.org/mediamanager.html#media
The problem is that the webkitplayers does not play many formats, so there's a big chance it won't work with your media. This brings us to option 2:
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2. Use VLC outside OpenLP (requires transparent theme):
Open VLC and set it up to display on the display with the presentation, as described here: http://superuser.com/questions/87436/how-do-i-get-vlc-to-play-on-the-second-monitor-on-windows
Then open and play the media.
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Needless to say this is not optimal, and this is something we hope to improve in one of the next releases, though it won't be fixed in the upcoming 2.2 release.